I made one out of a old towel... cut it to whatever size you want, for extra support you can double it up and sew them together or you could probably use fabric glue. Then just cut holes in each corner.
I had suction cups to hold it up, but it fell often. I bought the 3M hooks that stick with some sort of sticky stuff and it hasn't fallen once.
I personally used 1 layer of towel, not doubled up for a full grown bearded dragon. It is getting ragged but it was cheap to make and will be easy to replace.
Thats an old photo, don't use those suction cups... buy the stick on 3M hooks